You can download the Numbers '09 User Guide (and the iWork Formulas and Functions User Guide-a reference for all of the functions supported by Numbers, with syntax and descriptions) through the Help menu in Numbers. The Table of contents would lead you to the article 'Autofilling Table Cells' on page 78.

PS: An excellent place to have started your search would have been Chapter 4, Working with Table Cells, in the Numbers '09 User Guide. Numbers interprets cell references using only the column letter as meaning either the cell in that column on the same row as the cell containing the formula, OR the range of all cells in that column, depending which interpretation is appropriate to the formula. You could also replace your formula with this version, using the multiplication operator: range variable, unmerges the cells, then fills the range with the value of the first cell in the unmerged range (what the value was). If they are, it stores the range of the merged cells in a temp. Go to the Insert menu and choose Fill > Fill Down. This macro will search every cell in the active sheet to see if they are merged. Select G10 and all of the cells below in which you wish to use the formula. Numbers will adjust the cell reference to fit. Grab the control (small circle at the bottom right of the selection border) and drag it down as many rows as you wish to have the formula.

Am feeling very fragile and very fed up of this now.